Capacitive pressure transducers are widely used. In one known type of capacitive transducer, two planar layers of a conductive material are held substantially parallel to each other, separated by a narrow dielectric gap. The conductive layers are normally formed as coatings on respective diaphragm members. In a single-diaphragm transducer, one member is rigid, whereas the other is able to flex. The space between the members may be evacuated or may be at a reference pressure, and the test pressure, that is, the pressure to be measured, is applied to the outer side of the flexible diaphragm member. The greater the test pressure, the more the flexible member will flex toward the rigid member, and the greater the capacitance of the members will be. By comparing the capacitance of the members with a reference capacitance, the test pressure may then be determined.
Single-diaphragm capacitive pressure transducers suffer from several drawbacks. First, flexure of the one diaphragm member due to inertial and gravitational forces is indistinguishable from flexure due to actual pressure differentials over the members. Second, the sensitivity of single-diaphragm transducers is less than if both diaphragm members were allowed to flex.
In response to the shortcomings of single-diaphragm transducers, numerous double-diaphragm transducers have been developed. U.S. Pat. No. 3,858,097 (Polye, Dec. 31, 1974) and U.S. Pat. No. 4,422,125 (Antonazzi et al., Dec. 20, 1983) describe representative double-diaphragm transducers, in which conductive coatings are applied to two opposing, parallel, flexible diaphragm members. A reference pressure is created in the dielectric space between the two members The effect of gravity and of inertial forces is greatly reduced in these double-diaphragm devices since both diaphragm members will be affected approximately equally, with mainly only pressure differentials causing the members to move toward or away from each other.
The useful range of these known transducers is limited, however, by the fact that the reference pressure is fixed between the two diaphragm members. Increasing the stiffness of the members (for example, by making them thicker) makes it possible for the diaphragm to withstand and measure higher pressures above the reference pressure, but the sensitivity of the device decreases. Increasing the flexibility of the members increases sensitivity, but not only decreases the measurable pressure range (since, above a certain pressure, the members will touch and short-circuit, or will crack) but also makes the members more vulnerable to damage. What is needed is a pressure transducer which provides great sensitivity in pressure changes about a reference pressure which is externally variable.
The object of this invention is to provide a double-diaphragm pressure transducer which is easy to manufacture, which has improved sensitivity relative to the prior art, and which enables both the reference pressure and the test pressure to be varied easily.